Thanks Huddersfield Tiling. As this thread went dead I actually opened up a new one
Which backerboard for this? - https://www.tilersforums.com/threads/which-backerboard-for-this.99632/#post-979058
You're right, I'm surprised it lasted so long. Grout was rock hard and tiles never moved yet as I've stripped them off it looks like maybe the plaster wasn't primed, ridges not collapsed and the coverage was only about 40 - 50% hlding the tiles on.
The tray is rock solid. and looking up from the ceiling below there is a 2mm gap between the tray side and the wall. The water was coming in through the wall and not down the side of the tray. So I'm happy that's ok.
My first thought was the Silicon seal too, but covering the wall and leaving the seal exposed stopped the leak. I even over did the Silicon for a couple of weeks just to make sure.
When I stripped the tiles off I saw a few dap spots on halfway up the wall too where the grout line was.
Its all dried out now. Tiles have been off a couple of weeks. Going to board with waterproof board - not cement board as I don't see the point of cement board and tank.